I. Field of the Invention
The present invention relates generally to frequency synthesizers, and particularly to voltage-controlled ring oscillators for high-speed communications.
II. Description of the Related Art
Ultra-wide band (UWB) systems offer exceptionally high throughput, which provides an attractive solution for very high data rate, short-range wireless networks used in personal computer and consumer electronic applications. The Federal Communications Commission (FCC) regulations for UWB communications specifies an operating range of 3.1-10.6-GHz frequency while employing at least 500 MHz of bandwidth (measured at 10-dB points) with a power spectral density (PSD) of less than 41.25 dBm/MHz.
In one aspect of the related art, the multiband orthogonal frequency-division multiplexing (MB-OFDM) specification for ECMA368-369 uses orthogonal frequency-division multiplexing (OFDM) modulation and specifies up to 14 bands, each with a bandwidth of 528 MHz and with center frequencies that range from 3432 MHz to 10296 MHz. The bands are grouped into six band groups, such as shown in FIG. 1.
There are several demanding requirements that make a frequency synthesizer for an MB-OFDM transceiver significantly different from well-known synthesizer designs used in narrow-band wireless systems. For example, it is difficult to implement a standard phase-locked loop (PLL)-based synthesizer for very high frequencies. Furthermore, since the range of frequencies spans several gigahertz, multiple frequency sources are typically employed, such as described in A. Medi and W. Namgoong, “A fully integrated multi-output CMOS frequency synthesizer for channelized receivers,” in Proc. IEEE Int. Systems-on-Chip Conf., September 2003, pp. 75-78, B. Razavi, T. Aytur, F.-R. Yang, R.-H. Yan, H.-C. Kang, C.-C. Hsu, and C.-C. Lee, “A 0.13 μm CMOS UWB transceiver,” in IEEE Int. Solid-State Circuits Conf. Tech. Dig., February 2005, pp. 216-217, R. van de Beek, D. Leenaerts, and G. van der Weide, “A fast-hopping single-PLL 3-band MB-OFDM UWB synthesizer,” in IEEE J. Solid-State Circuits, July 2006, pp. 1522-1529, and J. Lee and D. Chiu, “A 7-band 3-8 GHz frequency synthesizer with 1 ns band-switching time in 0.18 μm CMOS technology,” in IEEE Int. Solid-State Circuits Conf. Tech. Dig., February 2005, pp. 204-205.
Therefore, there is a significant need in the art for a synthesizer that provides improved performance and reduces power consumption and enables a completely integrated MB-OFDM UWB transceiver.